A lithium secondary battery has been broadly used as a power source for a portable electronic device, such as a notebook computer and a cell phone, owing to its high discharge voltage and energy density. Further, development thereof as a power source for a hybrid electric vehicle, a plug-in hybrid electric vehicle, and an electric vehicle car is under way. The key issue is extension of the mileage per battery charge of a plug-in hybrid electric vehicle or an electric vehicle, and also for an HEV (hybrid electric vehicle) weight reduction and output increase of a storage battery has been demanded.
There are two possibilities for increasing the energy density (Wh/Kg=VAh/Kg) of an onboard lithium battery, namely to increase the capacity (Ah/Kg) of an electrode active material and to increase the discharge voltage (V), and it has been desired to realize the two. While, in a lithium cobalt oxide positive electrode currently used, only about a half of lithium is utilized by charging a battery at 4.2 V, and therefore if the utilization factor of the lithium ion can be enhanced by increasing the potential, the energy density can be improved.
Although heretofore an electrolyte for a lithium ion secondary battery has been constituted of a liquid electrolyte in which a lithium salt is dissolved in an aprotic organic solvent, or a gel polymer electrolyte impregnating the same into a porous structural material such as PVDF-HFP (porous polyvinylidene fluoride-hexafluoropropylene), the use of a flammable organic solvent has more serious safety problem, when a battery scale increases, and therefore a nonflammable or flame retardant electrolyte solution has been demanded.
For example, an inorganic solid electrolyte is a highly safe fireproof electrolyte. An oxide type and a sulfide type material having high ionic conductivity (order of magnitude of 10-3 S/cm) and electrochemical stability have been reported. However, since they are inorganic materials, they are brittle and formation of a cell is difficult. There is another drawback of poor contact between an electrode and an electrolyte.
The polymer electrolyte can be classified to a pure polymer electrolyte (hereinafter written as “polymer electrolyte”) and a gel electrolyte.
A polymer electrolyte is an electrolyte in which a lithium salt is dissolved in a host polymer such as polyethylene oxide. Since a battery using a polymer electrolyte is an all-solid-state type, there is no risk of liquid leakage, and therefore it is highly safe. However, the temperature dependence of the ionic conductivity of a polymer electrolyte is large, and the ionic conductivity at room temperature is a little bit too low (order of magnitude of 10−4 S/cm) and at a lower temperature below 0° C. it drops so much that it is also difficult to operate the battery.
On the other hand, a gel electrolyte is an electrolyte, in which a polymer is swollen by an organic electrolyte solution, and its ionic conductivity (order of magnitude 10−3 S/cm) is higher than that of a polymer electrolyte. Further, the interface resistance with an electrode is low, and a battery using a gel electrolyte is already on a stage of practical utilization or commercialization. However, by reason of use of an organic solvent, it is less safe compared to a polymer electrolyte.
While, an ionic liquid electrolyte is an electrolyte in which a lithium salt is dissolved in a molten salt with the melting point below room temperature. Although an ionic liquid electrolyte has high ionic conductivity, it has drawbacks to be overcome in electrochemical stability at a negative electrode, low temperature properties, and high cost.
As an electrolyte solution exhibiting high electrochemical stability even at a high potential, there is an electrolyte using a solvent containing fluorine and a solvent having a cyano group. For example, a fluorine-substituted carbonate ester electrolyte solution exhibits oxidation resistance as high as approx. 6 V. There is, however, a drawback of decrease in the solubility of a lithium salt in a solvent containing fluorine.
A Lewis acidic boron compound has a function of trapping an anion, and promotes dissociation of a lithium salt to enhance ionic conductivity. Further, it has been known that a boron compound has a flame retardant effect. Consequently, a boron compound has been used for a lithium salt containing boron, as well as for an electrolyte solution and for a polymer electrolyte. A boric ester, which is one of the most popular boron compounds, has been applied to an electrolyte solution, and, for example, suppression of increase in the interface resistance of an electrode by a mixture of a boric ester and an organic electrolytic system, or suppression of deterioration due to storage at a high temperature (Japanese Patent Application Laid-Open No. 2003-132946, and Japanese Patent Application Laid-Open No. 2003-317800), and enhancement of safety by suppression of flammability (Japanese Patent Application Laid-Open No. 2002-334717, and Japanese Patent Application Laid-Open No. 2008-300125), have been proposed.